Boolean modifier is giving intersection instead of difference
You need to unify the normal vectors of your parabola extrusion and then the boolean will display correctly.



To unify the normal vectors:


Select the parabola extrusion in the 3D view
Hit Tab to enter edit mode
Hit A to select all of the mesh
Hit CtrlN to recalculate the normals of the outsides
This makes all of the faces point in the same direction which is necessary for Boolean operations (and a lot of other stuff) to work correctly.
Hit Tab again to exit edit mode.  You'll need to hide the 'Para' object to see the effect as shown.